LEDs are light-emitting devices using semiconductors as the light-emitting material. The principle of emitting light is to apply an external voltage to enable electrons and holes in semiconductors to recombine and thus emitting photons. Since no filament is required, LEDs own the features of mercury free, saving power, low power consumption, small size, long lifetime, fast response, and high light-emitting efficiency and avoid the drawbacks of huge heat generation and burnout. Accordingly, LEDs are called the fourth-generation light source or the green light source.
The wavelength of the light emitted by an LED is determined by the energy level of the fabrication material and can cover the wavelength range from the ultraviolet light, visible light, to the infrared. Infrared LEDs can be applied in night monitor and optical fiber communication. Visible-light LEDs include multiple colors, and are applied extensively in general lighting, indicators, backlights, and plant lighting. Ultraviolet LEDs can be further classified into three segments of wavelength: UVA, UVB, and UVC. The wavelength of the UVA light is between 320 and 400 nm and the UVA light is also called near ultraviolet light. It can be applied in identification of counterfeit money, ultraviolet light therapy, and air purification. The wavelength of the UVB light is between 280 and 320 nm and the UVB light is applied in biotech industries and medical health protection. The ultraviolet light having wavelengths less than 280 nm is the UVC, also called the deep ultraviolet light. The UVC has the strongest energy among the three types, capable of penetrating the cell membrane and the cell nucleus of microorganisms and destroying the molecular bonds of nucleic acid. It is mainly applied in water or air sterilization systems. Currently, the applications and technologies of UVA and UVB have been mature. The related industries and research institutions keep developing the applications of UVC and breaking through the limitations of current technologies.
The current packaging technologies for ultraviolet LED are mainly the transistor outline can (TO-can) architecture. It owns excellent airtightness, and hence reducing the influence of the external environment on ultraviolet LED dies. In addition, the TO-can packaging technology is performed using inorganic materials. The aging problem caused by long-term illumination of ultraviolet light will not occur, leading to superior reliability. Nonetheless, owing to the existence of pins in TO-can metal packages, the heat dissipating capability is limited and the leads become the bottleneck for thermal conduction efficiency. Consequently, the TO-can technology is limited to low-power ultraviolet LEDs. In addition, the TO-can is a nonplanar packaging architecture, which imposes more spatial limitations on process backend modules and system integration.
The surface-mount device (SMD) packaging technology is a packaging technology for visible LEDs according to the prior art. It owns the advantages of small size, large angles of dispersion, and uniform light emission. The spatial limitation problem can be solved by replacing the TO-can packaging technology with the SMD packaging method. Nonetheless, the SMD technology adopts organic high-polymers, such as silica gels, acrylics, or epoxy resins, for packaging. Under the illumination of ultraviolet light, the chemical bonds of the high-polymers will be destroyed. The ultraviolet packages adopting organic high-polymers as the sealing material according to the prior art all face the problems of insufficient reliability and material aging. In order to overcome the drawbacks in the technologies according to the prior art, the present invention improves the SMD package structure according to the prior art. In addition to keeping the advantages of the SMD structure according to the prior art, aging of the packaging materials can be prevented. Besides, the limitation of planar packages according to the prior art can be conquered as well.